Optical emission, far infrared, Raman, resonance Raman, and Brillouin spectroscopy are being used to measure vibrational frequencies in a variety of systems as a function of variables that lead to phase transitions or conformational changes. The interatomic and intermolecular potentials, and changes in them, can be determined from these measurements. The experiments thus provide information about the microscopic mechanisms behind phase transitions or conformational changes in molecular systems. BIBLIOGRAPHIC REFERENCES: Direct Observation of a Soft Phonon Associated with a Structural Phase Transition in a Molecular Crystal, David M. Hanson, J. Chem. Phys. 63, 5046 (1975). Effect of the Exciton Bandwidth on Electron-Phonon Coupling in Molecular Crystals, David M. Hanson, Chem. Phys. Letters 43, 217 (1976).